Automatic gain control (“AGC”) circuits are electronic amplification circuits that are used to maintain a constant output signal level in the face of widely varying input signal levels. This is accomplished by automatically adjusting the gain of the circuit based on the input signal level. These circuits typically use some form of feedback from the output to control the amplification level applied to the input signal. In the electrical domain, the power level is controlled with a variable gain amplifier (“VGA”). In the optical domain, the optical power level is controlled with a variable optical attenuator (“VOA”).
Such circuits are used, for example, in optical receivers such as may be found in optical data communication network devices. In order operate at peak efficiency and at a minimum error rate, it is desirable to control the input power, both in the optical and electrical domains, to a level that is optimized for components and circuits downstream in the receiver. In other words, the received optical input signal must be conditioned to an optimized optical level and then again in the electrical domain to provide an optimized electrical output power level to downstream receiver components. Of course, such conditioning and optimization finds uses in many other implementations, and applicability to an optical receiver is merely exemplary.
While AGCs are generally known, existing circuits are often to slow to react to changes in the input signal. The result is that the output signal level becomes unstable and varies. The speed with which an AGC can adjust to changes in the input is referred to as “tracking bandwidth”. If the tracking bandwidth is too small, the AGC is not able to maintain a constant output signal under dynamic input conditions. If the tracking bandwidth is too high, the output can become unstable due to instability in the feedback loop of the AGC. This problem can be particularly detrimental in high speed optical networks where even a brief variance in the output signal can impair the receiver's ability to properly recover and extract the received information. Component variation can impact the tracking bandwidth.
It is readily understood that not all components used in an electrical or optical circuit exhibit the exact same performance characteristics due to tolerance variations. For example, a particular integrated circuit type used as a component in an automatic gain control circuit can vary from one integrated circuit to the next. The result is that, although the same integrated circuit may be specified in an AGC design, the variances and tolerances among components can result in inconsistent performance from one AGC to another. Similarly, as components age, their performance characteristics can change. Using the integrated circuit example, as an integrated circuit in an AGC ages, the change in performance of the integrated circuit results in a change in performance and tracking bandwidth of the AGC as a whole.
In addition, temperature can impact tracking bandwidth because changes in operating environment temperature can impact the performance of circuit components. Because an AGC can be implemented in systems that may find installation in widely varied environments, it is desirable that the AGC maintain a constant tracking bandwidth regardless of the temperature of the operating environment. For example, an optical receiver may be exposed to an operating environment that is very hot in the summer and very cold in the winter. While technicians can be dispatched to periodically measure the performance of the AGC and then manually recalibrate the circuit, such manual effort is expensive, time consuming and often occurs after performance of the AGC has degraded. As such, it is desirable to have an AGC that can automatically adjust to variances in circuit components to maintain a constant tracking bandwidth, regardless of whether the variances result from the aging of those components over time or variances among components of a particular type from one component to another and regardless of the operating environment temperature.
In addition to variances in AGC circuit components impacting tracking bandwidth, input signal noise can also impact the performance of an AGC. As such, in addition to having to detect and adjust to ACG circuit component performance changes, whether due to aging, inconsistency among components or temperature, such detection often must be made in the face of input signal noise. As such it is also desirable to have an AGC that can maintain a constant tracking bandwidth in the face of input signal noise, whether electrical signal noise or optical noise such as may be present in optical receivers.